The invention is directed to a process and apparatus for coating fuel, fertile material and/or absorber material containing particles with pyrolytic carbon and/or pyrolytic carbides by introducing thermally cleavable gases into the hot reaction space of a fluidized bed unit at temperatures above 1000.degree. C. Such particles are inserted into fuel elements or absorber elements as fuels or as absorbers for neutron absorption, which fuel elements or absorber elements are inserted in nuclear reactors, particularly in high temperature reactors.
Fuel elements for high temperature reactors generally consist of carbon as the structural material in which there is introduced the fuel and fertile material in the form of coated particles. These coated particles are spherical small particles of carbides and/or oxides of fuel and/or fertile materials, especially of uranium and thorium, also called heavy metal kernels which are coated with layers of pyrolytic carbon, sometimes also with layers of silicon carbide (J. L. Kaae, Journal of Nuclear Materials 29 (1969), 249-266).
In a given case coated absorber particles also are introduced into the fuel elements or into particular absorber elements. The coated absorber particles have a nucleus consisting of barium carbide or other boron compounds, e.g., borides, or other absorber compounds, e.g., hafnium carbide.
The production of the coated particles generally takes place by coating the heavy metal kernels in fluidized bed units. For this purpose the kernels are heated at a high temperature in a vertically standing graphite tube which is closed at the bottom with a conical or flat shaped perforated or fritted bottom. Carrier gas, usually argon is blown in through the bottom and so the particle charge is held in motion. The cleavable gas, e.g., a hydrocarbon gas, necessary for the coating is sometimes directly blown in through holes in the bottom of the bed, but is usually introduced through water cooled nozzles consisting of a nozzle tip with an elongated inlet tube, which is fitted into the bottom of the bed. The hydrocarbon is pyrolytically decomposed in the hot fluidized layer of the heavy metal kernels whereby the carbon is deposited as a layer on the particles and the hydrogen is removed with the waste gas (P. Koss, Ber. der Deutschen Keramischen Ges. 43 (1966), No. 3, pages 239-245).
Besides hydrocarbon gases there have also been employed other thermally cleavable gases in order to deposit other materials as coating on the kernels. Thus for the production of pyrolytic silicon carbide coatings there is generally used trimethyl chlorosilane and for depositing zirconium carbide coatings, zirconium chloride is employed. These thermally cleavable gases generally are diluted with an inert gas for the production of a suitable reaction result. The inert gas simultaneously serves in the fluidized bed as the carrier gas or as a supplement to the additionally introduced carrier gas for fluidizing the fluidized bed.
Besides the introduction of coating gases through the bottom of the fluidized bed recently good coating results have also been produced by introducing the coating gases into the fluidized layer from above via a water cooled lance of nozzles (German Offenlegungsschrift No. 2 343 123 and related Huschka U.S. application Ser. No. 500,017 filed Aug. 23, 1974 and now U.S. Pat. No. 4,056,641). The entire disclosure of the Huschka U.S. application is hereby incorporated by reference and relied upon.
In order to guarantee a trouble-free progress of the coating process the coating gas must be introduced into the fluidized bed below its decomposition temperature, since otherwise the gas inlet openings quickly clog up. The coating temperature in the fluidized bed is above 1000.degree. C., usually at about 1200.degree. to 2000.degree. C., and the gas inlet nozzle is in direct thermal contact with the solid likewise hot bottom of the reaction tube. FIG. 1 shows an illustrative form of such a prior art water cooled gas inlet nozzle. As a rule the gas inlet nozzle is made of metals whose melting point is below the coating temperature. As an exception customarily there is only the nozzle tip which, e.g., is made of molybdenum.
In such a fluidized bed the gas inlet nozzles assume the following functions.
They must center the reaction tube 2 with the bottom 1 in the hot tube 3, carry the weight of the reaction tube 2, the bottom 1 and the fluidized bed 4, guarantee a sufficiently tight sealing of the reaction space between the head 5 of the nozzles and the bottom 1 so that it is possible to introduce the carrier gas via the annular gap 6 in the fluidized bed 4 and the introduction of the coating gas, in a given case also the coating gas-carrier gas mixture, into the hot fluidized bed without inadmissably high heating of the coating gas, which depends on a sufficient removal of contact and radiation heat.
Furthermore, such gas inlet nozzles generally have an inner gas inlet tube 8 for the coating gas which is surrounded by the carrier gas inlet tube whose outer surface is cooled with the help of the conduit pipe 10 for the cooling water. Externally the gas inlet nozzle is closed by the metal outer jacket 9.
In the hitherto customary constructions for gas inlet nozzles these functions can be completely assumed so long as there is provision for a sufficient removal of heat. Because of the very high specific thermal loading per unit of surface between the hot reaction tube bottom and the head 5 of the nozzles it was hitherto believed a sufficient cooling could only be produced with water. The use of other cooling media had little success.
A particular danger for the previous fluidized bed furnace units, particularly for the gas inlet nozzle, is if the cooling water provision fails since the amount of heat stored in the hot furnace parts (reaction tube, bottom, hot tube, fluidized materials) is sufficient to heat the gas inlet nozzle up to the region of the melting temperature even if immediately after the failure of the cooling water the furnace heating is disconnected.
Furthermore in the production and processing of nuclear fuels, as is known, the nuclear physically permissible amounts of fissile material which can be handled in a container or apparatus of arbitrary geometry, the so-called safe amount, is greatly limited by the presence of a moderator, e.g., water. In fluidized bed furnace units with water cooled gas inlet nozzles there must always be reckoned with the danger of a water break and the flooding of the fissile material with water. Therewith, the per charge coatable amount of heavy metal kernels is limited to a specific size by the water cooling, which in the previously stated geometry by the coating process otherwise is only dependent on the type and composition of the heavy metal.
To avoid this limitation it has recently been proposed to employ in place of water as the fluid cooling medium carbon compounds containing chlorine and fluorine which are used many times in cooling and climate control. However, these materials are basically poorer heat conductors than water and have the disadvantage that they are thermally decomposed to a certain extent at the high temperature present. A further disadvantage is that these materials form decomposition products because of the impossibility of entirely excluding leakage in the hot reaction space, which act corrosively on the apparatus parts located in the waste gas tract. The danger of corrosion is particularly injurious in units in which fuels and fuel elements are produced in a reprocessing plant operated at a distance from the fissionable fuel in a high temperature reactor and obtained from fertile material and subsequently worked up, since in such a plant all maintenance operations are very difficult and expensive. In reprocessing plants there is the additional disadvantage that the chlorine-fluorine containing cooling media also decomposes by the radioactive radiation of the fluidized material.
Therefore, it was the problem of the invention to coat particles for the production of fuel elements and/or absorber elements for nuclear reactors by introducing thermally cleavable gases without their premature decomposition into the hot reaction space i.e., about 1000.degree. C., of a fluidized bed unit with the help of a gas inlet nozzle cooled with a cooling medium and having an elongated inlet tube without cooling which prevents a premature decomposition of the gases bringing with it a substantial limitation on the amount of fuel kernels added because of the nuclear physically safe conditions or the danger of a corrosive effect by the cooling medium or its decomposition products.